Fuel cell is a device without storing ability for converting chemical energy to electric power directly. When the loading is increased, output voltage of fuel cell is then decreased. Thus, a DC to DC converter is necessarily to dispose between the fuel cell and the loading for providing a stable voltage source to the loading and increasing the application fields. Traditional DC to DC converter is faced with limitation of low power converting efficiency and narrow operation range in design and manufacture. For achieving a holding time period in input voltage decreasing circumstance, operation cycle and other operation parameters should be tuned for achieving the requirement of the holding time period in low input voltage condition. On the other hand, the power converting efficiency will be sacrificed for gaining the input voltage for normal operation. The way aforementioned wastes electric power because it can not optimize the circuit design and select a normal input voltage range of resonating network parameters adaptively.
For example, when the DC to DC converter is practiced by a full-bridge or half-bridge construction, a LC resonating loop is used for assisting a semiconductor switch turning on without power loss or turning off. If the switching frequency is higher than the resonating frequency, the switching loss will be canceled when the switch is conducted in zero voltage condition. The primary disadvantages of which are that a larger frequency range is required for tuning output of a large load and the output can not be well tuned in zero load condition. Besides, the full-bridge converter causes the switch generating a surge voltage between drain electrode and source electrode due to inductance loss of the switching moment of the switch and imbalance of the switch characteristics. The surge voltage decreases the circuit stability and reliability.
It is desirable, therefore, to provide a circuit for avoiding surge voltage and switching loss for solving problems aforementioned.